This invention relates to methods and apparatus for delivering liquid precursors to semiconductor processing reactors.
There is a good discussion of various prior art proposals for delivering such precursors in the preamble of U.S. Pat. No. 5,620,524 and the reader is referred to this. In general a liquid precursor will be a gas at the low pressures, which typically exist in the reactor (e.g. 1 torr), but at atmospheric pressure (15 torr) they will be in their liquid state. This gives rise to practical delivery and measurement problems, which are exacerbated by the highly reactive nature of many of these precursors, which limit the choice of engineering materials that may be used. Most liquid precursor delivery systems, which are used in wafer processing, are of one of the following three types:
1. There is vaporisation of the liquid and the use of a gas mass flow controller.
2. Use of a liquid mass flow controller.
3. A carrier gas is passed through a liquid in a xe2x80x9cbubblerxe2x80x9d to vaporise some of the liquid in the carrier gas bubbles.
Whilst gas mass flow measuring instruments are widely used, they present problems when the liquid has to be vaporised for mass flow measurement. A large reservoir of vaporised liquid is required, which increases the residence time of the vaporised liquid at a raised temperature. For peroxide, which is a useful precursor, decomposition may commence before it reaches the process chamber. Liquid mass flow controllers are capable of giving good flow control but are liable to instabilities, believed to be caused by bubbles forming in the mass flow controller whilst in its idle state and the cold liquid cooling the controller can cause calibration drift. Bubblers require a carrier gas and the lower the vapour pressure, the less liquid that is taken up as vapour by the bubbles. Therefore, for low vapour pressure liquids, a high flow of carrier gas is required and that may be incompatible with the process.
As alternatives to these approaches, an arrangement similar to that shown in U.S. Pat. No. 5,620,524 has been tried comprising a positive displacement pump in combination with a narrow bore outlet pipe, but as well as difficulties in selecting appropriate materials for such pumps and other parts, flow variations are experienced due to variations in bore of the narrow bore pipe, necessary to buffer the low pressure chamber from the pump. Such arrangements are susceptible to variations because the flow restriction of the narrow bore pipe dominates as the pump is essentially only a pressurisation device. The results of this method are referred to as xe2x80x9cstandardxe2x80x9d in FIG. 3 below.
A simpler version of this system can be used if a pre-set or fixed flow is acceptable. The system then entirely depends on the action of ambient pressure upon the liquid to replace the use of the pump and the flow restricting tubing to control flow. This system makes use of the fact that clean rooms are pressure controlled environments at slightly above atmospheric pressure. The system is still however dependent on the internal bore diameter of the pipe being accurate and in suitably compatible materials this is not often the case. Specific system calibration can therefore become necessary and different pipe lengths may need to be used depending on the actual bore diameter. The results of this method are referred to as xe2x80x9cfixed flowxe2x80x9d in FIG. 3 below.
U.S. Pat. No. 5,620,524 describes an alternative using out-of-phase piston pumps, but it relies on the system knowing various characteristics of the precursor and monitoring the temperature and pressure of the precursor in order to deliver a desired mass of precursor.
U.S. Pat. No. 5,098,741 describes the use of a positive displacement pump to feed a liquid precursor to a CVD chamber, but the system requires the use of a pressure metre controlling a variable orifice valve to overcome the problem of dissolved gases.
From one aspect the invention consists in apparatus for delivering a liquid precursor to a semiconductor processing reactor comprising a source of a liquid precursor, a volume calibrated positive displacement pump drawing liquid precursor from the source via an input path and for delivering, along an output path, (preferably without any pressure control and/or pulsing) a known volume of liquid precursor to the reactor. Either just prior to the reactor or within the reactor the liquid precursor may be evaporated, for example by a flash evaporator.
The apparatus may further include a valve for connecting the pump to the input path or the output path. The apparatus may further comprise control means for controlling the pump and the valve whereby the pump draws liquid in, whilst the reactor is not processing with this precursor. Additionally or alternatively the apparatus may further include control means for controlling the pump and the valve whereby the pump returns any undelivered precursor to the source. This recucling, particularly when combined with the pump only being charged when needed, significantly reduces the presence of dissolved gases.
The pump may be in the form of a syringe pump.
The source may include another reservoir of liquid precursor and preferably the source may include at least two bottles or reservoirs and means for automatically connecting one bottle or reservoir to the inlet path as the other becomes empty.
The apparatus preferably includes a linear encoder controlled stepper motor for driving the pump to deliver the known volume.
From another aspect the invention consists in a method of delivering a liquid precursor to a semiconductor processing reactor including:
1. Delivering a liquid precursor, along an input path, into a volume calibrated positive displacement pump from a source of liquid precursor;
2. Delivering, along an output path, preferably without pressure control and/or pulsing a known volume of liquid precursor to the reactor.
The liquid precursor may be evaporated either just before it reaches the reactor or within the reactor.
This may further include permanently connecting the pump to the input path and the output path. Additionally or alternatively, after each volume delivery, the pump may return any remaining liquid to the source, before drawing a fresh charge.
Although the invention has been defined above it is to be understood it includes any inventive combination of the features set out above or in the following description.